1. Field of the Invention
The present invention relates to a motor formed in an ultra-compact shape.
2. Related Background Art
A motor formed as a compact motor is conventionally well known as, for example, a compact cylindrical step motor shown in FIG. 7. A stator 102 of the step motor is formed in such a manner that a stator coil 105 is coaxially wound around a bobbin 101, the bobbin is stationarily sandwiched between two stator yokes 106 in the axial direction, stator teeth 106a and 106b are alternately placed on the stator yoke 106 in the circumferential direction of the bobbin along the inner surface thereof, and that the stator yoke 106 integrated to the stator tooth 106a or 106b is fixed to the case 103.
A flange 115 and a bearing 108 are fixed to one of two cases 103 and another bearing is fixed to the other case. A rotor 109 is composed of a rotor magnet 111 fixed to a rotor shaft 110. The rotor magnet is rotatably supported between two bearings 108.
A mechanism has been proposed in which a leadscrew is formed on the rotor shaft 110, and the leadscrew engages with a female thread (not shown) to move the female thread in the axial direction. The mechanism is used in, for example, an autofocus mechanism of a video camera.
However, since, in the above-mentioned conventional step motor, the case 3, bobbin 101, stator coil 105, and the stator yoke 106 are coaxially placed on the outer periphery of the rotor, such a disadvantage that the outside dimension of motor becomes large arose. Further, since magnetic flux, which is generated by energization to the stator coil, is mainly passed through an end surface 106a 1 of a stator tooth 106a and an end surface 106b1 of a stator tooth 106b as shown in FIG. 8, it does not effectively act on a rotor magnet 111. Accordingly, the output of motor was not increased.
A motor that solved such problems is known in U.S. Pat. No. 5,831,356. The motor is formed in such a manner that a permanent magnet rotor divided into equal parts in the circumferential direction and alternately polarized into different poles is formed in a cylindrical shape, a first coil, the rotor and a second coil are placed in the axial direction of the rotor, a first outer magnetic pole and a first inner magnetic pole excited with the first coil are opposingly placed on the outer periphery and the inner periphery of the rotor, respectively, and a second outer magnetic pole and a second inner magnetic pole excited with the second coil are opposingly placed on the outer periphery and the inner periphery of the rotor, respectively. A rotary shaft that is a rotor shaft is extended from within the cylindrical permanent magnet.
Thus formed, the motor may have a high output and a small outside dimension. Nevertheless, if, by thinning the magnet in such configuration the distance between the first outer magnetic pole and the first inner magnetic pole and the distance between the second outer magnetic pole and the second inner magnetic pole are decreased, the magnetic resistance of a magnetic circuit, may be decreased. Accordingly, current which is passed through the first coil and the second coil enables the generation of increased magnetic flux in a reduced amount of current.
Further, when a conventional motor shown in FIG. 7 is applied to such a mechanism that a leadscrew is formed in the output shaft, the leadscrew is allowed to engage with a female thread to move the female thread in the axial direction, pressurization of the output shaft or rotor shaft in the axial direction to bias the output shaft or the rotor shaft in the axial direction is needed so that a hysteresis difference is not generated. This pressurizing means is usually a plate-shaped spring and is placed on the rear end surface of the motor body. Therefore, a length the entire motor including the pressurizing means becomes longer and the compactness of motor is lost. Further, looseness between the rotor shaft and the bearing portion generates a larger working noise.